This invention relates to fuel cell systems and, in particular, to integrated fuel cell and fossil fuel power plant systems having enhanced efficiency.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten carbonate fuel cells operate by passing a reactant fuel gas through the anode, while oxidizing gas comprising carbon dioxide and oxygen is passed through the cathode.
Fossil fuel power plants produce energy by combusting fossil fuels, such as coal or natural gas. As a result of the combustion process, fossil fuel power plants generate flue gas, which is often disposed of by atmospheric emissions. Such emissions, however, are harmful to the environment because they contain large amounts of carbon dioxide which contributes to global warming and climate changes.
Accordingly, a number of approaches have been used to control or limit carbon dioxide content in the flue gas emissions from fossil fuel power plants. However, the process of separating the carbon dioxide from the flue gas is not cost effective because of the low concentration of carbon dioxide (approximately 10%) in the gas.
A system is disclosed in U.S. Pat. No. 5,232,793 in which carbon dioxide emissions in the flue gas of a fossil fuel power plant are reduced by using a carbonate fuel cell in tandem with the power plant. In this system, the flue gas is added to an oxidant supply and the combined gas is used as the feed gas for the cathode of the molten carbonate fuel cell. The electrochemical reaction in the fuel cell then results in the carbon dioxide in the feed gas being transferred from the cathode to the anode of the fuel cell. The anode exhaust gas thus becomes highly concentrated with carbon dioxide gas. This permits the carbon dioxide gas to be efficiently separated form the exhaust gas and to then be disposed of or converted to a form where it can put to a useful purpose.
The system of the '793 patent uses an externally reforming fuel cell system. The external reformer generates flue gas. For efficient CO2 recovery, the reforming fuel also needs to be recycled to the fuel cell oxidant supply line. In addition, this invention adds the ambient temperature air required by the fuel cell to the fossil fuel power plant exhaust. This requires further preheating of the oxidant feed to the fuel cell to realize the required operating temperature. The separate air addition and external reforming of '793 both add complexity and cost to the system. They also reduce the efficiency of the system.
It is therefore an object of the present invention to provide an integrated power production system comprised of a fossil fuel power plant and carbonate fuel cell which attempts to alleviate the aforesaid disadvantages.
It is a further object of the present invention to provide an integrated power production system comprised of a fossil fuel power plant and carbonate fuel cell which is less complex and has improved efficiency.